1. Field of the Invention
The invention relates to medical imaging devices and more specifically to an electrical power supply of an X-ray tube and especially an electrical power supply of an X-ray computed tomography system.
It also relates to industrial applications, such as the X-ray checking of luggage in airports, enabling a differentiation of the density and the nature of the objects observed.
2. Description of Related Art
Computed Tomography (CT) is an X-ray medical imaging process which makes to possible, using a plurality of two-dimensional images (2D) acquired about an object or a patient to be imaged, to obtain a three-dimensional image (3D) of the object or the patient.
Throughout the acquisition, therefore at high frequencies (approximately 1 to 10 kHz), it is sometimes desirable to change the nature of the X-rays particularly to image a patient or an object in a contrasted manner.
As is known per se, the nature of the X-rays is particularly changed by modifying the power supply voltage of the X-ray tube between two levels named kV+ and kV−.
It must be possible to make such a change as quickly as possible by switching the power supply voltage of the X-ray tube rapidly from a first voltage to a second voltage. Such switching must for example be performed between 10 μs and 30 μs.
For example, for a switching time of 20 μs, this is equivalent to one tenth of the acquisition period, taking for example an acquisition frequency of 5 kHz.
However, the high voltage power supply of the X-ray tube comprises a filtering capacitor, whereto the parasitic capacitor Cp of the high voltage cable is added (for a single-pole tube and per polarity in the case of a bipolar tube).
When said capacitor is discharged, by the current consumed by the tube, this results in a transition time from kV+ to kV− depending on said current and which is frequently prohibitive.
For example, for voltages kV+=140 kV and kV−=80 kV, a capacitor is 500 pF, and the current consumed is 600 mA. The resultant transition time from kV+ to kV− is equal to 50 μs.
In FIG. 1, a diagram illustrating the high voltage cable 10 is represented, wherein the entire high voltage capacitor has been symbolically allocated to Cp (filtering capacitor plus parasitic capacitor), the X-ray tube 11, the power supply A supplying both high voltages kV+ and kV−.
If it is required to discharge said capacitor Cp more rapidly than in the tube, this generates energy which needs to be dissipated. Recharging also requires that the generator return said energy with the same transition time. This renders the power supply more complex.